This invention relates generally to limited slip differentials. More particularly, this invention relates to limited slip differentials having a retarding differential action mechanism that may be activated by an external signal.
Vehicles use a differential to drive axles of parallel wheels at essentially the same time. Vehicles include automobiles, trucks, and the like. The differential permits the wheels to rotate at different speeds, such as when the vehicle makes a turn. The differential permits the outer wheel to turn faster than the inner wheel during the turn. A limited slip differential usually is used to divert more driving force to a wheel with traction when the other wheel begins to slip. A limited slip differential typically has a clutch mechanism to limit or retard the differential action between the wheels and transfer more of the available torque to the wheel with traction. A limited slip differential usually has a gear case enclosing a differential gear set, which has a pair of pinion gears and a pair of side gears. A clutch typically is positioned between one of the side gears and the gear case. When engaged, the clutch retards the relative rotation between the gear case and the side gear. Other limited slip differentials may have other arrangements and configurations of the clutch with the gear set.
Many limited slip differentials have an actuating mechanism to engage and disengage the clutch. In some limited slip differentials, the clutch is actuated in response to the sensation of a predetermined speed differential between the side gears. In other limited slip differentials, the clutch is actuated in response to an external input signal. The actuating mechanism in many limited slip differentials is a ball-ramp actuator positioned outside or within the wall of the gear case.
A ball-ramp actuator usually has an internal actuating plate disposed within the differential housing and an external actuating plate disposed outside to the differential housing. Each of the actuating plates has corresponding raceways for holding cam balls between the actuating plates. The raceways are bi-directional, forming a curve and an incline in an axial direction. As the cam balls roll in the raceways, the linear axial direction of the cam balls changes in response to the incline. Upon activation, a solenoid or other electromagnetic device attracts and frictionally engages a friction material on the external actuation plate. The frictional engagement of the external actuating plate retards the rotation of the internal actuating plate, thus rotating the cam balls in the raceways and xe2x80x9cupxe2x80x9d the incline and causing the clutch to engage. When deactivated, the solenoid or other electromagnetic device disengages from the external actuating plate. The cam balls rotate in the raceways and xe2x80x9cdownxe2x80x9d the incline, causing the clutch to disengage.
The friction material on the external actuating plate can wear-out prematurely. The friction materials also may change friction characteristics due to heat and misalignments. The friction material may have higher costs from special materials and construction. The frictional material may have a relatively smaller engagement area with the solenoid or other electromagnetic device. A smaller engagement area would require a higher engagement force to activate the ball-ramp actuator.
This invention provides a limited slip differential system where an eddy current activates a retarding differential mechanism such as a ball-ramp actuator and a clutch assembly.
In one aspect, a limited slip differential system has a clutch assembly, a ball-ramp actuator, a rotor, and a coil assembly. The ball-ramp actuator is connected to the clutch assembly. The rotor is connected to the ball-ramp actuator. The coil assembly is disposed adjacent to the rotor. The coil assembly and the rotor form a gap. The coil assembly is operable to generate one or more eddy currents in the rotor. The eddy currents are operable to create a braking torque in the ball ramp actuator.
In a further aspect, a limited slip differential system includes a clutch assembly, a ball-ramp actuator, a rotor, a coil assembly, and an electric control unit. The ball-ramp actuator is connected to the clutch assembly. The rotor is connected to the ball-ramp actuator. The coil assembly is disposed adjacent to the rotor. The coil assembly and the rotor form a gap. The electric control unit is connected to the coil assembly. The coil assembly is operable to generate one or more eddy currents in response to an electric power signal from the electric control unit. The eddy currents are operable to create a braking torque in the ball ramp actuator. The electric control unit is operable to actively monitor an electromotive force from the coil assembly.
A further aspect provides a method for activating a limited slip differential having a ball-ramp actuator an d a clutch assembly in a differential case. In this method, one or more eddy currents are induced in the ball-ramp actuator. A braking torque is generated in the ball-ramp actuator in response to the eddy cur rents. The clutch engages the differential case in response to the braking torque.